One of the problems of semiconductor manufacturing is making contact to the various elemdents of an integrated circuit. It is desirable for the contact area to be as small as possible and still make effective electrical contact. In the case of a MOS transistor, the contact to the source/drain requires that the source and drain areas be made sufficiently large to accommodate the contact. The area is significantly larger than is otherwise required for source/drain functionality. There is also the problem of making contact to the gate. The gates of many of the transistors in a typical integrated circuit are of the minimum gate length of that can be successfully drawn for the particular process that is being used. The contact hole may be even larger than that minimum dimension. Also there is an alignment problem. The misalignment between the gate and the contact hole is typically in the range of plus or minus 10% of the minimum gate length. Thus, if the contact is made over the active region where the transistor is formed, the contact hole will be over the source or drain as well as the gate due to misalignment. Due to etching processes, the etch for the contact hole will typically extend to the source or drain. Thus, an attempt to form a contact to gate over the active region will result in shorting the gate to the source or drain. Thus, contacts to gate are made over a field region. This extends the gate unnecessarily and increases the amount of area for transistor layout.
Another characteristic common to MOS integrated circuits is field oxide defining the active region. The field oxide is grown after the active region has been masked. The result is a field oxide which, at the active region boundary, is sloped. This field oxide at the active-region boundary has become commonly known as a bird's beak. This is an area where many defects occur due to numerous different problems. Additionally, the field oxide provides isolation between transistors in adjacent active regions. The isolation is greater for thicker field oxide but the bird's beak is an area which is quite thin. One of the solutions that has been proposed includes subsequent formation of an insulator with vertical walls inside the active region defined by conventional field oxide. This is described in an article, "A Super Self-Aligned Source/Drain MOSFET", C. K. Lau et al., IEDM-87, 1987 IEEE, pages 358-361. This, however, creates a large wast of space due to the oversized active region to accommodate the subsequently formed vertical-wall field oxide within the previously defined active region.